Isolates of staphylococcus aureus and S. epidermidis were examined for penicillin sensitivity or resistance using the following criteria: 1) MIC, 2) inhibition zone size, 3) nature of the zone edge, and, 4) penicillinase assay. Results indicate that the MIC's and nature of the zone edge are good predictors of resistance although a small per cent of isolates with "sensitive" MIC's were found to be penicillinase producers. Absolute zone size criteria showed most "Intermediate" zones to be penicillinase producers.